1. Technical Field
The present invention relates to an improved bearing preferable for sustaining the rotational portion such as a spindle motor for office automation equipments like computers and peripheral devices thereof.
2. Description of the Prior Art
The bearing apparatus is used for sustaining a rotational portion such as a motor for driving a magnetic disk or disks or sustaining a pivotal swing arm portion of a hard disk drive as peripheral devices for computers. As shown in FIG. 20 the bearing usually includes a pair of single row type ball bearings. A spacer 34 is interposed between outer rings 32b, 33b of upper and lower ball bearings 32, 33 having inner rings 32a, 33a fit and attached to a shaft 31. The reference numerals 32c, 33c depict balls, and 35 depicts ball retainers.
In order for the bearing apparatus to sustain a rotational load or a pivotal load with sufficient stiffness and to secure a rotational accuracy or a pivotal accuracy, the inner rings 32a, 33a are applied predetermined pressure (hereinafter pre-load) and secured to the shaft by adhesion under ambient temperature.
However, upon an increase in the temperature of the bearing from ambient temperature under the effect of the frictional heat generated by the rotational operation or the pivotal operation of the bearing itself or the effect of heat supplied from outside the bearing, components of the bearing expand respectively into different dimensions. In case of the ball bearing 32 and 33 of the bearing apparatus, the order of an amount of the relation of expansion of the components in the radial direction is:
the outer ring greater than the inner ring greater than balls.
There is a following relationship between the radial clearance and the dimension of each components of the bearing. (radial clearance)=[inner diameter of the outer racewayxe2x88x92(2xc3x97diameter of each ball+outer diameter of the inner raceway)]
In this connection, upon an increase in temperature from the ambient temperature, an amount of enlargement of the inner diameter of the outer raceway will be larger than the enlargement of the outer diameter of the inner raceway formed around the inner ring, and the clearance between the two raceways will also be enlarged. Whereas an amount of the enlargement of the balls is small relative to the inner and outer raceways so that the radial clearance will be enlarged upon an increase in the temperature from the ambient temperature. The enlargement of the radial clearance will cause a range of a predetermined characteristic oscillation frequency to vary upon rotation of the bearing apparatus, and in some cases there results a resonance with other components of the equipment into which the bearing apparatus is incorporated.
When the bearing apparatus of the prior art of the above described structure is used in the rotating portion of a motor for driving a hard disk drive, sometimes accuracy of writing or reading data is hampered by an unacceptable level of vibration of the drive caused by resonance with the other component, such as a swing arm portion, a chassis or a baseplate. There is also a possibility for generating noises caused by the vibration, and reducing the quietness of the drive.
Provided that the difference of the amount of thermal expansion between the clearance of the inner and outer rings and the balls is further increased, a clearance is defined between the balls and the raceways of the inner and outer rings, and the rotational inaccuracy of a hub of the motor and the run out of the surface of the magnetic disk or disks due to the rotational run out of the hub of the motor is increased. This effect will deteriorate the reliability of the hard disk drive.
Although the balls are usually made of steel material, ceramic material may also be used for enhancing the durability thereof. In such a case, the above mentioned problem caused by the difference of the amount of thermal expansion between components will become more serious, since the amount of thermal expansion of the ceramic material is further lower (about {fraction (1/10)}) than that of the iron material used for the inner and outer rings.
Accordingly the object of the present invention is to provide a bearing wherein an appropriate radial clearance is maintained and thus the predetermined pre-load is applied constantly to the balls even if the components of the bearing expand by an increase in temperature from the ambient temperature thereof. The purpose of the present invention is to provide a bearing apparatus capable of sustaining high rotational accuracy or high pivotal accuracy and long life even upon increases of temperature from ambient temperature.
In order to achieve the purpose of the present invention, a bearing apparatus according to a first aspect of the present invention is formed by a plurality of balls arranged between an inner and an outer ring, and a ring made of a material with a lower factor of linear expansion than the outer ring material is press fitted around the outer ring.
A compound bearing appratus in accordance with the second aspect of the present invention comprises
a shaft to which an inner ring is fit slidably, a cylindrical outer ring member surrounding the shaft, a plurality of balls of a first row interposed between a first inner raceway formed on an outer periphery of the inner ring and a first outer raceway formed on an inner periphery of the outer ring member, a plurality of balls of a second row interposed between a second inner raceway formed directly on an outer periphery of the shaft and a second outer raceway formed on the inner periphery of the outer ring member, the bearing apparatus being characterized in that it comprises:
a low expansion ring press fit around the outer periphery of the outer ring member, wherein the low expansion ring is made of a material with a lower factor of linear thermal expansion than a material of the outer ring member, and the inner ring is secured on the shaft with applying a reasonable amount of pre-load thereon.
A compound bearing apparatus in accordance with the third aspect of the present invention comprises
a stepped shaft having a larger diameter shaft portion and a reduced diameter shaft portion, an inner ring fit slidably around the reduced diameter shaft portion of the stepped shaft, and a cylindrical outer ring member surrounding the shaft, a plurality of balls of a first row interposed between a first inner raceway formed on an outer periphery of the inner ring and a first outer raceway formed on an inner periphery of the outer ring member, and a plurality of balls of a second row interposed between a second inner raceway formed directly on an outer periphery of the larger diameter shaft portion of the stepped shaft and a second outer raceway formed on an inner periphery of the outer ring member, the bearing apparatus being characterized in that it comprises:
a low expansion ring press fit around the outer periphery of the outer ring member, wherein the low expansion ring is made of a material with a lower factor of linear thermal expansion than a material of the outer ring member, and the inner ring is secured on the shaft with applying a reasonable amount of pre-load thereon.
The balls are made of ceramic material and the low expansion ring is also made of ceramic material.